As is known, modem fitted sinks can be made of a thermosetting composite material by means of a mass that contains a resin which constitutes the matrix and a filler material that is mostly formed by mineral particles.
As disclosed for example in European patent EP 361101, the filler material is constituted, for approximately 50-85% by weight of the mass, by mineral particles constituted by natural quartz sand.
Other known solutions use cristobalite, which is obtained from quartz particles crystallized beforehand in a high-temperature furnace.
In both cases, the particles have a size from 0.1 to 2 mm.
The resulting articles, particularly fitted kitchen sinks, have on their exposed surface a weave-like structure that is substantially caused by the fact that during polymerization the resin shrinks more in its surface regions, which in the case of sinks are also the parts that are subject to wear.
Therefore, substantially, on the surface which is ultimately the surface that is used, many small hollows form between the filler particles that lie closer to the surface.
These articles, though being manufactured so as to have a scratch-resistant surface, may be subject to wear especially in the regions where intense abrasion occurs due to rubbing of pots, cutlery and the like, such as the bottom of the sink and the surface of the draining board.
A problem that is encountered with these articles is that the mineral particles at the surface are covered only by a thin layer of resin, which in practice renders the particle impermeable and prevents direct contact with liquids.
Over time, especially in the regions that are most affected by abrasion, the thin layer of resin is worn, thus placing the mineral particles in direct contact with the liquids that are present in the sink, such liquids being usually highly stain-forming, such as for example coffee, tomato juice, tea, and so forth.
Quartz particles are inherently porous and cristobalite is even more porous, since it is obtained by baking quartz in a high-temperature furnace, and this causes many microperforations in the quartz particles, making them even more porous and prone to absorb liquids in their mass.
When the mineral particles are no longer protected by the thin layer of resin, they inevitably absorb liquids and anything else that is present in the sink; this rapidly causes stains on the most heavily abraded surfaces of the sink.
Moreover, the quartz particles that remain exposed also act as an ideal medium for the deposition of organic material, thereby providing a microenvironment suitable for bacterial proliferation.